The invention relates to a controlled equilibrium device, and more specifically the invention relates to a controlled equilibrium device that provides active relative position control between first and second members where the controlled equilibrium device comprises a first spring rate applied when the device is below a threshold displacement value, a second spring rate applied when the device is above a threshold displacement value and integral damping for controlling the device displacement.
Conventional passive isolators and mounts include elastomeric or other resilient members producing a support stiffness that limits, for a particular environment or application, the transmission of vibratory disturbances and displacements across the isolator. If the input excitation frequencies are well above the suspension resonance, then good isolation can be achieved without undue difficulty. However, often there are also large inputs with frequencies near the suspension resonance, which amplifies the excitation. There may be insufficient damping in the conventional passive isolators to control motion at resonance. In such case, the well known isolators with resilient members typically comprise a trade-off between low or xe2x80x9csoftxe2x80x9d spring rates suitable for effectively limiting the transmission of high frequency vibratory inputs, and high or xe2x80x9chardxe2x80x9d spring rates suitable for limiting the relative motion across the isolator caused by low frequency inputs. If the spring rate is too hard, excessive high frequency vibration will be transmitted. If the spring rate is too soft, low frequency inputs will produce excessive strain in the resilient members causing durability problems. In addition, if the spring rate is too soft, low frequency inputs will produce damaging collisions between the suspended device and adjacent devices, due to the large relative displacements. Conventional passive isolators may incorporate progressive stiffness such that the mounts snub near end-of-travel for large, low-frequency input excitations. This is only a slight improvement, since isolation will be degraded for all times at the higher stiffness, and contacting the snubbers typically results in a jolt with resulting decreased passenger comfort or reduced life for suspended components. For the soft spring rate, conventional passive isolators must accommodate a large static deflection, which causes increased size, weight and cost, which are undesirable. If the static load size and location varies, the resulting static deflection also varies, which increases difficulty for connecting to adjacent unsuspended devices and precludes maintaining a level platform which reduces passenger comfort such as in truck cabs.
The above risk of excessive strain in the resilient member may be decreased by increasing the size of the rubber elements, thereby not increasing the strain even as the displacement is increased. However, the increased size will result in greater size, weight and cost, which are undesirable. The above risk of damaging collisions may be decreased by utilizing increased damping, particularly at resonance. However, elastomers typically have low damping values, and those with high damping have reduced durability and higher compression set, which in turn increases the static deflection which is undesirable as discussed above. Adding external damping, for example hydraulic shock absorbers, may help but at increased complexity and cost. Even if the size of the rubber elements and damping could be increased without added size, weight and cost, the resulting system would still have the aforementioned difficulties with snubbing and load leveling.
An alternative to the conventional passive isolator is the conventional passive air-suspension with separate load-leveling valve and hydraulic damper. The air-suspension provides a soft suspension for good isolation of high frequency inputs. The load leveling valve limits the static deflection thereby reducing strain in the suspension. This system effectively provides a high spring rate at very low, quasi-static response frequencies, and a soft spring rate at all other frequencies. The load leveling for a conventional passive air-suspension typically includes a xe2x80x9cdead bandxe2x80x9d wherein the valve is not actuated for high frequency, low amplitude vibrations, thereby minimizing power consumption. The hydraulic damper allows greater damping than the conventional passive isolator and thus limits motion near resonance. However, hydraulic dampers are velocity-sensitive devices, and thus provide no damping at very low, quasi-static response frequencies. Thus the conventional passive air-suspension with separate load-leveling valve and hydraulic damper are particularly ill-suited for applications wherein high damping at very low frequency, quasi-static roll and pitch are important for driver comfort and perceived safety, such as in a four-point-soft cab suspensions. In addition, the load leveling provided by a conventional passive air-suspension responds too slowly to react to anything but quasi-static inputs. Thus, the load leveling provided by a conventional passive air-suspension cannot reduce the time spent near end of travel at the snubbers with resulting reduced performance as discussed above. If the reciprocal of the response time is the xe2x80x9cbreak frequencyxe2x80x9d, then the xe2x80x9cbreak frequencyxe2x80x9d must be less than the resonance frequency to maintain stability. For this system, adding damping allows a faster stable response time. By carefully tuning and controlling damping, the response time can be optimized and controlled in a stable manner, thereby providing xe2x80x9cfast load levelingxe2x80x9d. Fast load leveling further improves the vibration isolation, since it optimally reduces the amount of time near the xe2x80x9cend of travelxe2x80x9d where the high stiffness of the snubbing reduces the isolation. However, the hydraulic dampers used in a conventional passive air-suspension are not easily controlled since they are sensitive to temperature and prone to leakage. It is therefore desirable to integrate the air-suspension and leveling valve into a single device with carefully tuned, well controlled damping. In addition, integrating the air-suspension, load-leveling valve and damping into a single device reduces system complexity, eliminates device support bracketry and thereby reduces system size, weight and cost.
Another means for canceling noise and vibration across a broadband of frequencies is through the use of active mounts such as those described in U.S. Pat. Nos. 5,526,2292 and 5,845,236 both assigned to Lord Corporation. Such active devices use actuators to cancel the force that is transmitted through the mount. In effect, active mounts are very soft at relatively high vibration frequencies and reasonably stiff at low frequencies to control relative motion. Active mounts provide suitable vibration control however active mounts are quite expensive and conventional active mounts and systems require complex electronic control systems and methods.
Control equilibrium devices or CEM""s are typically soft mounts that include a load leveling feature which makes the mount stiffer at low frequencies and soft at high frequencies. One such CEM is disclosed in United Kingdom Patent Number 2,298,021 for xe2x80x9cImproved Vibration Isolatorxe2x80x9d issued and assigned to Barry Controls Limited. As shown in the FIGS. 2 and 4 of the ""021 reference, the vibration isolator generally comprises a core assembly movably suspended within a housing by a resilient member and a valve arrangement such that when the core assembly reaches a predetermined point of deflection, the valve arrangement causes fluid to flow into the core to apply a force tending to return the core assembly to the equilibrium position.
In the ""021 reference the core assembly translates freely through the housing until it contacts or is snubbed by the housing. No damping is provided except the low-level damping in the elastomeric members, which as discussed above for conventional passive isolators, may be insufficient to control motion at resonance. Thus, when the device is excited at resonance, the core unit will experience large amplitude displacements and likely will snub out a number of times. The snubbing is undesirable and may decrease passenger comfort, decrease the life of the mounted device, and limit the effective useful life of the mount. It would be desirable to provide damping to the movable core assembly to control the assembly displacement and further limit snubbing contact between mount component parts during resonance and other frequencies at or near resonance. Damping may also be desirable for stability and speed of response.
The foregoing illustrates limitations known to exist in present rubber mounts. Thus, it is apparent that it would be advantageous to provide an alternative controlled equilibrium mount directed to overcoming the limitations set forth above. Accordingly, a suitable alternative mount is provided including features more fully disclosed hereinafter.
In one aspect of the present invention this is accomplished by providing a controlled equilibrium mount or suspension device that provides displacement dependent spring rates, active relative position control and load leveling during resonance and during low frequency high amplitude inputs. The mount of the present invention also provides integral damping during mount displacement. The suspension device, comprises: a housing comprising a wall that defines a housing chamber, the housing wall comprising an integral valve including an inlet for supplying displacement means to the chamber and a discharge port for discharging the displacement means from the chamber; at least one spring, each at least one spring having a spring stiffness; and a load leveling device supported by one of at least one spring, the load leveling device comprising a member movable through the housing chamber by the displacement means, the load leveling device comprising a first deadband displacement zone defined between the inlet and a portion of the movable member and a second deadband displacement zone defined between the discharge port and a second portion of the movable member, the movable member being at a nominal position when the movable member displacement is within the deadband zones, the integral valve being closed to the supply and discharge of displacement means when the movable member is in the nominal position; and wherein when the movable member is displaced out of either deadband zone the integral valve is open to provide the required change in displacement means to return the movable member to the nominal position.
Integral damping is provided to control the displacement of a movable mass such as a truck cab. The integral damping may be comprised of surface effect damping. The mount may include a movable piston member movable through the mount housing. The desired surface effect damping is provided as at least three protuberances along the outer surface of the piston sealingly and frictionally engage a resilient layer of material along the housing chamber wall during relative displacement of the piston and resilient layer.
The load leveling device is generally comprised of means for sensing displacement of a movable member. If the movable member of the device has experienced displacement that exceeds the deadband range as disclosed in the preferred embodiments of the invention, the sensing means provides a signal either electrically by an electrically actuated device such as a switch or sensor or mechanically by as a valve to indicate such displacement. The load leveling device also comprises a controller for controlling the return displacement of the mount in order to ensure that the mount is returned to its desired nominal orientation. The controller may be comprised of an electrically or mechanically actuated device. Finally the load leveling device comprises a displacement source for returning the mount to its nominal configuration and such source may comprise a motor, air, hydraulic fluid or a thermally expandable phase change material such as wax. Depending on the applied loads and mount design, the displacement device may be comprised solely of air or hydraulic fluid or may be comprised of such in combination with a discrete spring member Kc such as a mechanical spring or an elastomer member. The movable load leveling member may be comprised of a piston, shaft or a movable plate. As will be described hereinafter, when the mount experiences large, low frequency loads, the movable member typically travels through the deadband zone and the load leveling device serves to return the mount to its nominal orientation.
The suspension device of the present invention has a spring rate that is greater during periods where low frequency high amplitude vibrations are applied to the device than during periods where high frequency low amplitude vibrations are applied to the device.
The present invention may be used in a variety of applications including but not limited to supporting a passenger cab for a truck, a vehicle engine, a wing mounted aircraft engine, a fuselage mounted aircraft engine or a pylon in a helicopter rotor.
In summary, the device of the present invention provides vibration control characteristics of a soft mount with effective motion control; provides broadband, high frequency control, has the potential for a longer useful life than conventional rubber mounts, provides improved isolation and also provides active relative position control and load leveling features.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.